Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
In the fuel cell, a gas chiefly containing oxygen or the air (hereinafter also referred to as the “oxygen-containing gas”) is supplied to the cathode, and a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the “hydrogen-containing gas”) or CO is supplied to the anode. The oxygen-containing gas and the fuel gas used in the reaction are disposed as an exhaust gas.
Since the exhaust gas contains the unconsumed fuel gas which has not been burned in the reaction, it is not economical to dispose all fuel gas including the unburned fuel gas as an exhaust gas. In this regard, as a technique for reducing the amount of the unburned fuel gas disposed of wastefully, for example, a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2002-151106 is known.
As shown in FIG. 11, the fuel cell is formed by stacking thin disks 1 and ring plate separators 2 alternately. The disk 1 is made of solid electrolyte material or the like. Ring plate path separators 3, 4 are formed on both surfaces of the disk 1.
A cell reaction passage 5a is formed between one surface of the disk 1 and one separator 2 through the path separator 3, and an air passage 5b is formed between the other surface of the disk 1 and the other separator 2 through the path separator 4.
A through hole 6 forming a fuel gas passage is provided at the center of the disk 1, and a plurality of combustion gas passages 7 are provided around the through hole 6. The through hole 6 is connected to the inlet of the cell reaction passage 5a through distribution inlet holes 8a, and the outlet of the cell reaction passage 5a is connected to the combustion gas passages 7 through discharge holes 8b. 
According to the disclosure, fuel gas flows along the through hole 6, and then, the fuel gas is supplied into the cell reaction passage 5a through the respective distribution inlet holes 8a. The fuel gas makes a U-turn in the outer region of the disk 1, and flows into the combustion gas passages 7 from the discharge holes 8b. Then, the unburned fuel gas which is discharged to the combustion gas passages 7 is supplied into the through hole 6 of the fuel cell (not shown) connected to the combustion gas passages 7 on the downstream side, and used again for reaction. Thus, complete combustion of the fuel gas is achieved.
In the conventional technique, the through hole 6 is formed at the center of the disk 1, and combustion gas passages 7 are formed around the through hole 6. Therefore, the process of fabricating the disk 1 is considerably complicated, and special seal structure is required. Further, the shapes of the separator 2 and the path separators 3, 4 are complicated. Thus, the overall structure of the fuel cell is not economical.
Further, the fuel gas before consumption and the fuel gas after consumption are mixed together, and supplied to the cell reaction passage 5a. Therefore, density difference tends to occur in the fuel gas which is supplied to each of the cell reaction passages 5a in the stacking direction. Consequently, the power generation reaction occurs differently depending on the disk 1.